This invention relates to thermally conductive adhesives and, more particularly, it relates to an adhesive matrix filled with a three-dimensional arrangement of mesophase pitch based carbon fibers.
Thermal management in electronic packaging has always been an important design consideration, and it is even more so today because packaging density (and hence heat dissipation) is increasing dramatically. More powerful and closer spaced chips, finer grid patterns, and surface mount devices such as leadless ceramic chip carriers need materials exhibiting high thermal conductivities and/or tailorable coefficients of thermal) expansions (CTE), so that heat removal is efficient and/or CTE mismatches between components are reduced. Facilitating heat removal and reducing CTE mismatches are very desirable attributes for they lead to more flexible packaging design, improved operating performance, and increased life expectancy (through reduced failures).
Thermally conductive, tailorable CTE organic-based adhesives (in paste or film form) play an important role in thermal management in electronic packaging. They are used for a variety of applications, including:
die (chip) bonding to chip carriers, substrates or heat sinks
chip carrier and substrate attachment to printed wiring boards (PWB)
PWB bonding to heat sinks
heat sink attachment to chassis
lid sealing
resistor, capacitor, diode, etc. attachment (i.e. using fillet adhesives)
Typically these adhesives are filled with metallic fillers such as silver or inorganic fillers such as alumina or boron nitride. Alumina fillers, which are electrically insulating, increase the thermal conductivity of the base polymer approximately 3-5.times., based on the filler loading, geometry and size. Silver flakes are used when maximum thermal conductivity and electrical conductivity are required, as it too increases the thermal conductivity of the base polymer approximately 3-5.times., depending on the loading. All of these fillers have several drawbacks, however. High weight percentages of silver are required (e.g. 79 wt. % is used in some adhesives to achieve the desired performance levels), and since silver is expensive, the cost of these fillers becomes substantial. Similar to silver, high weight percentages of inorganic fillers are also required and are relatively expensive as well. Silver also suffers from a phenomena referred to as metallic electromigration; an article by S. Krumbein (IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 11, No. 1, Mar. 1988) describes metal migration in more detail.
It is known to use adhesives filled with carbon fibers for the purpose of making electrical connections in electrical systems bound together by the adhesive.